Field
The disclosed concept pertains generally to electrical switching apparatus, such as, for example, circuit breakers.
Background Information
Electrical switching apparatus are used to protect electrical circuitry from damage due to a trip condition, such as, an overcurrent condition, an undervoltage condition, a relatively high level short circuit or fault condition, a ground fault or an arc fault condition. Compact molded case circuit breakers (compact MCCBs), for example, include at least one pair of separable contacts which are operated either manually by way of a handle disposed on the outside of the case, or automatically by way of a trip unit in response to the trip condition.
Compact MCCBs include a thermal-magnetic trip mechanism. The thermal aspect of the trip mechanism includes a bimetal piece through which current flows. An increase in current causes the temperature of the bimetal piece to rise, which in turn causes it to bend. When the bimetal piece bends a sufficient amount, it causes the compact MCCB to trip. The thermal aspect of the trip mechanism provides a long delay trip function, which is often triggered by a sustained overcurrent condition.
The magnetic aspect of the trip mechanism includes a magnetic clapper structure through which current flows. When the current increases above a threshold level, a magnetic field induced by the current flowing through the magnetic clapper structure causes an associated cantilever to move. The movement of the cantilever causes the compact MCCB to trip. The magnetic aspect of the trip mechanism provides an instantaneous trip function.
Many types of circuit breakers include an electronic trip unit (ETU). The ETU receives input from one or more sensors, such as a current transformer (CT) to sense current, located either in a circuit breaker or outside of a circuit breaker and determines whether a fault condition occurs. The ETU can control whether to trip open the separable contacts of the circuit breaker. The ETU also allows users to modify trip settings, such as a current at which the circuit breaker will trip or the delay time before tripping the circuit breaker. Power to operate the ETU is provided by a CT disposed around a conductor in the circuit breaker or outside of the circuit breaker. The CT is also used to sense current flowing through the circuit breaker. A relatively large size CT is required to accurately sense current over a wide range.
Compact MCCBs are small and the existing components in prior compact MCCBs do not leave enough room to incorporate an ETU or its associated components, such as a CT. As such, ETUs have not been incorporated into compact molded case circuit breakers.
FIG. 1 is an isometric view of a conventional compact MCCB 100 and
FIG. 2 is a cross-sectional view of the compact MCCB 100 of FIG. 1. As shown in FIGS. 1 and 2, the compact MCCB 100 does not include an ETU or its associated components. Due to the limited space and configuration of components inside the compact MCCB 100, an ETU and its associated components, such as a CT, cannot be incorporated into the compact MCCB 100.
There is room for improvement in electrical switching apparatus.